You cannot simultaneously use a computer's wi-fi connection to connect to the Internet and share that connection to other devices. Likewise, you cannot simultaneously connect an iPhone to a wi-fi network and share that network as a hotspot.
As you've found, If you had a wired ethernet connection, you could use the computer's wi-fi to share that with your phone. Or, if your service plan allows it, you can share your phone's 3G connection through your phone's hotspot feature to your computer. But you cannot share a wi-fi connection over the same wi-fi interface.
So far as I know, the iPhone does not support using a Bluetooth internet connection from another device. Apple just does not enable that functionality. Again, some service plans allow you to share a phones 3G connection over bluetooth, but I don't believe it's possible with wi-fi on any iPhone.
If you had a second wi-fi interface (such as a USB wi-fi module) you could connect to the marina's network using that interface and then share it out using the AirPort interface. That is the only way this could be done.